"There has been concern for decades about possible damaging effects of high-protein diets on the kidney," said Dr. Allon Friedman, an associate professor of medicine at the Indiana University School of Medicine, Indianapolis. It was thought that excess protein "can rev up the filtering mechanism in the kidney, causing damage over time," he explained.

To see whether those fears were founded, Friedman and his team compared the effects on renal function of a low-carbohydrate, high-protein diet with those of a standard low-fat, calorie-restricted diet. For two years, they followed 307 obese men and women, half of whom followed the high-protein diet, and half assigned to the low-fat plan. None had kidney disease.

For the high-protein diet, the researchers used guidelines from "Dr. Atkins' New Diet Revolution," which limits carbohydrates while permitting unlimited fat and protein intake. In the low-fat group, women were instructed to consume 1,200 to 1,500 calories a day, and men, 1,500 to 1,800 calories daily. They ate about 55 percent of calories from carbs, 30 percent from fat and 15 percent from protein.

The researchers then measured markers of kidney function.

"For otherwise healthy obese people, we did not find that a low-carbohydrate, high-protein diet was dangerous to kidney heath or put people at increased risk for that two-year period," he said.

"One of the take-home messages is, the fact that you lose weight is more important than how you get there," Friedman added.

However, it is unknown whether the results would hold longer than two years or apply to people with chronic conditions such as diabetes, high blood pressure or kidney disease, Friedman said.

The study, supported by U.S. National Institutes of Health grants, is published online May 31 and in the July print issue of the Clinical Journal of the American Society of Nephrology.

Weight loss was similar for both groups. By 24 months, the dieters had lost about 7 percent of body weight on average.

"Both groups had a drop in the protein in their urine," Friedman said. Excess protein in the urine can indicate kidney problems.

Many participants dropped out before the end of the study. At two years, only 74 of the low-carb group and 83 of the low-fat group completed the blood test measurements.

Connie Diekman, director of university nutrition at Washington University in St. Louis, called the study "an interesting first look at an issue of longstanding concern, high-protein diets and renal function."

Still more research is needed, she said, including a study to compare a high-protein diet in healthy individuals free of kidney disease with those who have a family history of kidney problems.

Because the findings don't apply to those with kidney problems, "I would caution people with a history of renal [kidney] problems about the potential impact of high protein on the health of the kidney," Diekman said.

Diet plans should be developed on an individual basis, she said.

The authors said longer follow-up would help determine effects on kidney function over a longer period, and Friedman said he hopes to look at the effect of high-protein diets on those who have kidney disease.